Micromachined resonant magnetic field sensors

ABSTRACT

A micromachined magnetic field sensor is disclosed. The micromachined magnetic field sensor includes a substrate; and a drive subsystem partially supported by the substrate with a plurality of beams, and at least one anchor; a mechanism for providing an electrical current through the drive subsystem along a first axis; and Lorentz force acting on the drive subsystem along a second axis in response to a magnetic field vector along a third axis. The micromachined magnetic field sensor also includes a position transducer to detect the motion of the drive subsystem and an electrostatic offset cancellation mechanism coupled to the drive subsystem.

FIELD OF THE INVENTION

The present invention relates generally to sensors and more particularlyto micromachined resonant magnetic field sensors.

RELATED APPLICATIONS

U.S. Pat. No. 6,892,575, entitled “X-Y Axis Dual-Mass Tuning ForkGyroscope with Vertically Integrated Electronics and Wafer-ScaleHermetic Packaging,” issued May 17, 2005,

U.S. Pat. No. 6,939,473, entitled “Method of Making an X-Y AxisDual-Mass Tuning Fork Gyroscope with Vertically Integrated Electronicsand Wafer-Scale Hermetic Packaging,” issued Sep. 6, 2005,

U.S. Pat. No. 7,104,129, entitled “Vertically Integrated MEMSStructure,” issued Sep. 12, 2006,

U.S. Pat. No. 7,247,246, entitled “Vertical Integration of a MEMSStructure with Electronics in a Hermetically Sealed Cavity,” issued Jul.24, 2007,

U.S. Pat. No. 7,442,570, entitled “Method of Fabrication of Al/GEBonding in a Wafer Packaging Environment and a Product ProducedTherefrom,” issued Oct. 28, 2008,

U.S. patent application Ser. No. 12/184,231, entitled “Method ofFabrication of Al/GE Bonding in a Wafer Packaging Environment and aProduct Produced Thereupon,” filed on Jul. 31, 2008 and assigned to theassignee of the present invention,

U.S. patent application, Serial No. 2009019382, entitled “Dual ModeSensing For Vibratory Gyroscope, filed on Aug. 6, 2009 and assigned tothe assignee of the present invention; and

U.S. Patent Application, entitled “Micromachined Resonant Magnetic FieldSensors,” serial no. (IVS-144, 4976P), filed on even date herewith, andassigned to the assignee of the present invention,

all of which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Magnetic field sensors are widely deployed in consumer and industrialinstruments for applications varying from position sensing, currentsensing, data storage, and magnetic compassing. There are many methodsto sense magnetic fields including Hall-effect, magneto-diode,magneto-transistor, magnetoresistive-effect, magnetic tunnel junction,magneto-optical, fluxgate, search coil, and Lorentz force.

The Lorentz force resonant sensor fabricated by means of MEMS technologyis preferred due to its low-cost batch fabrication technology. Lorentzforce-effect resonant sensors are manufactured in a process flow similarto the process of motion sensors, such as accelerometers and gyroscopes.In addition, because Lorentz force-effect does not require specialmagnetic materials, it is the most compatible sensing mechanism for anintegrated platform of motion sensors; magnetic field sensors,accelerometers, and gyroscopes. However, to detect a magnetic field asweak as the earth magnetic field, the magnetic field sensor is desiredto have high sensitivity and low offset. High sensitivity is generallyachieved by exciting at the resonant frequency and being packaged invacuum for high quality factor. However, if the drive frequency is offby Δf from the resonant frequency, the sensitivity decreasessignificantly which proportional to Δf. In prior art, a close-loopfrequency control system is needed to adjust the driving frequencydynamically to prevent significant sensitivity variation and thecomplicated close-loop control system could consume as high as 1milliwatt per axis. In addition, the offset is generally larger than thesignal and the offset variation limits the minimum detectable signallevel. Thus, the design to mitigate offset is critical. Accordingly,what is needed is a system and method to address the above-identifiedissues. The present invention addresses such a need.

BRIEF SUMMARY OF THE INVENTION

A micromachined magnetic field sensor is disclosed. The micromachinedmagnetic field sensor comprises a substrate; and a drive subsystempartially supported by the substrate with a plurality of beams, and atleast one anchor; a mechanism for providing an electrical currentthrough the drive subsystem along a first axis; and Lorentz force actingon the drive subsystem along a second axis in response to a magneticfield vector along a third axis. The micromachined magnetic field sensoralso includes a position transducer to detect the motion of the drivesubsystem and an electrostatic offset cancellation mechanism coupled tothe drive subsystem.

In designing high sensitivity, high manufacturability, low cost, and lowpower consumption Lorentz force magnetic field sensors, the presentinvention addresses the need of:

1. High-manufacturability, low power consumption, and low Brownian noiseby establishing a open-loop dual-mode resonating devices;

2. High sensitivity and low cost fabrication by deploying electrostaticcoupling spring with high bias voltage at the sensing subsystem;

3. Low offset by deploying:

a) electrical shield structures comprising at least one electricalsource connected to at least one electrode to reduce the electrostaticoffset force along the second axis generated by parasitic capacitorsaround the drive subsystem;

b) the shield structure has a force-balanced characteristic with twoshield structures sandwiching the drive subsystem along the second axisto cancel the electrostatic offset force generated by the shieldstructures;

c) the voltage of the electrical source of the shield structures isequal to the voltage of the drive point of the drive subsystem tominimize the electrostatic offset force.

d) a mismatch-trimming actuator moving the micromachined magnetic sensoralong the second axis in order to reduce the gap mismatch of theforce-balanced electrostatic coupling springs;

e) an AC ripple voltage detector where the detection connects a drivepoint to at least one anchor for detecting the ripple of the drivepoint.

4. Low power consumption by having electrostatic self-test actuator.

By harnessing these design strategies, a Lorentz force resonant sensorcan be implemented in a way to allow for on-chip integration of magneticfield sensors with other motional sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a dual mode open-loop magnetometer system.

FIGS. 2A and 2B are a model of a dual mass system (a) before coupled and(b) after coupled.

FIG. 3 is the first transfer function of the dual mode open-loopmagnetometer system.

FIG. 4 is the second transfer function of the dual mode open-loopmagnetometer system.

FIG. 5 is the embodiment of a mechanical coupling spring.

FIG. 6 is the embodiment of an electrostatic coupling spring.

FIG. 7 is the embodiment of open-loop dual-mode Lorentz-force magneticsensing device with signal processing circuitry.

FIG. 8 is the embodiment of open-loop dual-mode Lorentz-force magneticsensing device for sensing in-plane magnetic fields.

FIG. 9 is an embodiment of open-loop dual-mode Lorentz-force magneticsensing device for sensing out-of-plane magnetic fields.

FIG. 10 is an embodiment of 3-axis magnetic sensing system with 3open-loop dual-mode Lorentz-force magnetic sensing devices.

FIGS. 11A-11C are embodiments of a self-test actuator with amagnetometer system (a) for a dual mode system; (b) an implementationwith electrostatic comb self-test actuators; (c) for a single modesystem.

FIGS. 11D and 11E are embodiments of a dual mode open loop system and asingle mode open loop system, respectively that includes currentcarrying coils.

FIGS. 12A-12C are embodiments of a stiffness trimming block with amagnetometer system; (a) an embodiment for a dual mode system; (b) animplementation with electrostatic parallel plate actuators as thestiffness trimming block; (c) an embodiment for a single mode system.

FIGS. 13A and 13B are embodiments of a shield structure with (a) a dualmode open-loop magnetometer system; and (b) a single mode open-loopmagnetometer system.

FIG. 14A-14C are embodiments of a dual mode open-loop magnetometersystem with a) a mismatch-trimming actuator (MTA), (b) an embodiment ofMTA, and (c) an MTA embodiment with a single mode open-loop magnetometersystem.

FIGS. 15A and 15B are embodiments of an AC voltage control system for adual mode open-loop magnetometer system with (a) resistive typedetection and (b) capacitive type detection.

FIGS. 16A and 16B are embodiments of self-test actuators, force-balancedshield structures, and mismatch trim actuators with open-loop dual-modeLorentz-force magnetic sensing devices for detecting (a) in-plane and(b) out-of-plane magnetic fields.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to sensors and more particularlyto micromachined resonant magnetic field sensors. The followingdescription is presented to enable one of ordinary skill in the art tomake and use the invention is provided in the context of a patentapplication and its requirements. The present invention is not intendedto be limited to the implementations shown but is to be accorded thewidest scope consistent with the principles and features describedherein.

An embodiment of an open-loop Lorentz-force magnetic sensing device isshown in FIG. 1.

System

The dual mode open-loop system 2 comprises a drive subsystem 60, a sensesubsystem 70 and a coupling spring 30, a position transducer 40, signalprocessing electronics 50, and drive electronics 80 for supplying acurrent flowing through a portion of the drive subsystem 60. The drivesubsystem 60 comprises a plurality of beams, such that in the presenceof a magnetic field, the drive subsystem 60 is actuated through theLorentz force proportional to the magnetic field. The coupling spring 30couples the motion and the force of the drive subsystem 60 and the sensesubsystem 70 resulting in at least two resonant modes: in-phase mode andanti-phase mode. The sense subsystem 70 comprising a plurality of beamsand converts the actuation force from the coupling spring 30 into outputmotion. The position transducer 40 detects the output motion of thesense subsystem 70 and may be designed with mechanisms comprisingparallel plate capacitive transducers, interdigitated comb capacitivetransducers, piezoresistive sensors, optical sensors, and piezoelectricsensors. The drive subsystem 60 and the sense subsystem 70 aremechanically anchored to a substrate 1 through plurality of beams,respectively.

Operation

FIG. 1 depicts the disclosed sensing method comprising flexibly-coupleddrive and sense subsystems 60 and 70. An AC current generated by thedrive electronics 80 passes through a plurality of beams 11 and 12 andestablishes a current vector in the first axis at the drive point 10. Adistributed Lorentz force is generated in the second axis in thepresence of a magnetic field vector in the third axis, and the totalforce isF _(Lorentz)=∫(I×B)dl  (1)where the I is the magnitude of AC current, B is the magnitude ofmagnetic field density, and dl is the unit length of the portion of theplurality of beams 11, 12 where AC current flows. The Lorentz forceactuates the drive point 10 along the second axis proportional to themagnetic field density.

A coupling spring 30 couples the motion of the drive point 10 to thesense point 20 and the sense subsystem 70 resulting in motion at thesense subsystem 70 along the fourth axis proportional to the magneticfield density. The motion at the sense subsystem 70 is detected by aposition transducer 40 and the electrical signals from the positiontransducer 40 are further processed by the signal processing unit 50.

FIG. 2A depicts the uncoupled mechanical model of a sense subsystem 170characterized by a sense resonant mode having frequencyω_(S)=√(k_(S)/m_(S)), and an uncoupled drive subsystem 160 characterizedby a drive resonant mode having frequency ω_(D)=√(k_(D)/m_(D)). Thesense subsystem 170 comprises mass m_(S) 140, a sense spring 120 withstiffness k_(S)−Δk, and the coupling spring 122 with stiffness Δk. Thedrive subsystem 160 comprises a mass m_(D) 130, a drive spring 110 withstiffness k_(D)−Δk, and the coupling spring 122 with stiffness Δk. Thesense mass m_(S) 140 is suspended from the substrate 101 by the sensespring 120 while the drive mass m_(D) 130 is suspended from thesubstrate 101 by the drive spring 110. Referring to FIG. 2B, the sensemass m_(S) 140 and the drive mass m_(D) 130 are flexibly coupled throughthe coupling spring 122 with stiffness Δk such that both stiffnessk_(S)−Δk and stiffness k_(D)−Δk are greater than zero, i.e. k_(S)−Δk>0and k_(D)−Δk>0. The sensing system from FIG. 2(B) yields a total of fourtransfer functions whose inputs are generalized forces F_(S) and F_(D)and outputs are motion of the particular mass, x_(S) or x_(D).

The relationship between the position of the sense mass m_(S) 140,x_(S), and force acting on the drive mass 130, F_(D), can be expressedas the following transfer function:

$\begin{matrix}{{G_{DS}(s)} = {\frac{x_{S}(s)}{F_{D}(s)} = {\frac{1}{m_{D}m_{S}}\frac{\Delta\; k}{( {s^{2} + \omega_{S\; 1}^{2}} )( {s^{2} + \omega_{S\; 2}^{2}} )}}}} & (2)\end{matrix}$where ω_(S1) 220 and ω_(S2) 230 as shown in FIG. 3 are modal frequenciesassigned to two vibratory modes of the system from FIG. 2(B). Similarly,the relationship between the position of the drive mass 130, x_(d), andforce acting on the sense mass 140, F_(S), can be expressed as a yetanother transfer function:

$\begin{matrix}{{G_{SD}(s)} = {\frac{x_{D}(s)}{F_{S}(s)} = {\frac{1}{m_{D}m_{S}}\frac{\Delta\; k}{( {s^{2} + \omega_{S\; 1}^{2}} )( {s^{2} + \omega_{S\; 2}^{2}} )}}}} & (3)\end{matrix}$

Further, the relationship between the position of, and the force actingon either drive mass m_(D) 130 or sense mass m_(S) 140 can be expressedwith the following transfer functions:

$\begin{matrix}{{G_{SS}(s)} = {\frac{x_{S}(s)}{F_{S}(s)} = {\frac{1}{m_{S}}\frac{( {s^{2} + \omega_{D}^{2}} )}{( {s^{2} + \omega_{S\; 1}^{2}} )( {s^{2} + \omega_{S\; 2}^{2}} )}}}} & (4) \\{{G_{DD}(s)} = {\frac{x_{D}(s)}{F_{D}(s)} = {\frac{1}{m_{D}}\frac{( {s^{2} + \omega_{S}^{2}} )}{( {s^{2} + \omega_{S\; 1}^{2}} )( {s^{2} + \omega_{S\; 2}^{2}} )}}}} & (5)\end{matrix}$where ω_(S) is the anti-resonance of the sense subsystem 170 and ω_(D)is the anti-resonance of the drive subsystem 160.

Based on Equation (2) and (5), the force in drive subsystem results inthe displacement of the drive subsystem which can be significantlysmaller than the displacement of the sense subsystem for forcingfrequency near ω_(S).

The advantages of dual-mode Lorentz-force magnetic sensing device aretwo-fold: stable sensitivity and low Brownian noise. Both of theseadvantages are described herewith.

Stable Sensitivity

Shown in FIG. 3, the first transfer functions 210 is the plot ofEquation (2) which is from forces at the drive subsystem 160 of FIG. 2Bto the displacement of the sense subsystem 170. In addition, it isproportional to the sensitivity of the dual mode open-loop system ofFIG. 2B. Because of the existence of two subsystems, the transferfunction 210 has at least two resonant peaks labeled as 220 and 230.Around the central frequency ω₀ 240 which are the root-mean-square ofthe frequencies of the two peaks 220 and 230, the gain of the transferfunction 210 is stable even with small frequency perturbation. It isdesired to achieve a stable gain by designing the frequency of the ACcurrent from the drive electronics 80 of FIG. 1 at the central frequencyω₀ 240. For an open-loop system, typically the frequency of the transferfunction shifts because of manufacturing-related and temperature-relatedvariations. The design of the dual mode system of FIG. 2B operatesaround the central frequency ω₀ 240 stabilizes the gain and mitigatesthe effect of manufacturing-related and temperature-related variations.

Low Brownian Noise

The second transfer function 310 is the plot of Equation (4) which isfrom forces at the sense subsystem 170 of FIG. 2A to the displacement ofthe sense subsystem 170 is shown in FIG. 4. The transfer function has ananti-resonance at notch frequency ω_(N) 350 corresponding to the drivesubsystem anti-resonance which can be designed to be close to thecentral frequency ω₀ 240. The total Brownian noise output is the sum inenergy of the Brownian noise output from the drive subsystem 160 andthat from the sense subsystem 170. The Brownian noise output is theproduct of the transfer function and the Brownian noise force. Ingeneral, the Brownian noise force at the sense subsystem 170 is largerthan the Brownian noise force at the drive subsystem 160. Byincorporating the anti-resonance of the transfer function 310, theBrownian noise output is minimized and the dual mode open-loop system 2can achieve low Brownian noise.

The coupling spring 122 is important to the operation of a dual modeopen-loop system. The frequency of two resonant peaks labeled as 220 and230 of the transfer function 210 may be calculated as:

$\begin{matrix}{{\omega_{220}^{2} = {\omega_{0}^{2}( {1 - \frac{{\Delta\; k}\;}{\sqrt{k_{D}k_{S}}}} )}}{\omega_{230}^{2} = {\omega_{0}^{2}( {1 + \frac{{\Delta\; k}\;}{\sqrt{k_{D}k_{S}}}} )}}} & (6)\end{matrix}$where k_(S), k_(D) and Δk are the stiffness of the sense subsystem 170,the drive subsystem 160, and the coupling spring 122 and this stiffnessdefine frequency separation between 220 and 230. Coupling stiffness Δkmay be substantially large in order to separate peaks, yielding widersensor bandwidth. On the other hand, coupling stiffness Δk may besubstantially small to keep peaks close to achieve high transducer gain.

The coupling element 122 can be implemented as structural couplingsprings or electrostatic coupling springs.

Structural Coupling Springs

An embodiment of a structural coupling spring 431 is shown in FIG. 5.The spring 431 includes folded beams where the stiffness of thestructural coupling spring 431 is defined by the structural width,length, height, and material properties. In some cases, the sense pointis preferred to be electrically isolated from the drive point 410. Forexample, the sense point 420 can be biased at high voltage to increasethe transduction gain of the position transducer 40 (FIG. 1) while thedrive point 410 is biased at a low voltage that is limited by the driveelectronics 80 (FIG. 1). In this case, the material of the structuralcoupling spring 431 can be designed to be highly resistive or anadditional dielectric layer can be inserted in the structural couplingspring 431. However, a complicated process with at least one extra maskis required to establish low resistance structures for the drivesubsystem 60 (FIG. 1) and high resistance structures or structures witha dielectric layer for the coupling spring 431.

Electrostatic Coupling Springs 30

Electrostatic coupling springs can be designed for different mechanicalmovements, i.e., in-plane movement or out-of-plane movement.

In-plane Electrostatic Coupling Springs:

An embodiment of electrostatic coupling spring 532 is shown in FIG. 6.In the case of in-plane motion of the drive point 510 referenced to thesense point 520 along the second axis, the electrostatic coupling spring532 includes electrostatic parallel plates with electrode gaps 532 a and532 b, respectively. The in-plane coupling stiffness Δk can be modeledas

$\begin{matrix}{{\Delta\; k} \propto {\frac{{- 2}ɛ\; A}{g^{3}}V^{2}}} & (7)\end{matrix}$

where the ∈, A, g, and V are the permittivity, electrode area, electrodegap, and bias voltage across the in-plane electrostatic coupling spring532. The polarity of the in-plane coupling stiffness indicates that thein-plane electrostatic coupling spring is a negative spring and thetotal stiffness and the resonant frequency are reduced.

Out-of-plane Electrostatic Coupling Springs:

The embodiment of the electrostatic coupling spring 532 shown in FIG. 6can be deployed for out-of-plane motion coupling. In the case ofout-of-plane motion of the drive point 510 referenced to the sense point520 along the third axis, the electrostatic coupling spring 532 acts asinterdigitated combs. A typical device can be built with height rangingfrom 10 microns to 100 microns and a typical gap can vary from 1 micronto 5 microns. For a structural height of 30 microns and the gap of 2microns, the out-of-plane coupling stiffness Δk can be modeled as

$\begin{matrix}{{\Delta\; k} \propto {n\;\frac{\pi^{3}}{20}\frac{ɛ\;{Lov}}{g^{2}}V^{2}}} & (8)\end{matrix}$where the n, Lov, g, and V are the number of combs, comb overlap length,electrode gap, and bias voltage across the out-of-plane electrostaticcoupling spring 532. The polarity of the out-of-plane coupling stiffnessindicates that the out-of-plane electrostatic coupling spring is apositive spring and the total stiffness and resonant frequency areincreased.

The advantages of electrostatic coupling springs are twofold. The firstadvantage is the high impedance at DC between the drive point 510 andthe sense point 520. Thus, the sense point 520 can be biased at highvoltage to increase the transduction gain of the position transducer 40(FIG. 1) without any complicated process mentioned in the section of thestructural coupling spring 431 (FIG. 5). The second advantage of theelectrostatic coupling springs is that a weak stiffness is easilyprovided. As mentioned before, that coupling stiffness Δk may besubstantially small to keep resonant peaks 220 and 230 of FIG. 4 closeto achieve high transducer gain. This could be problematic whenutilizing a structural coupling spring 431 which may require a long andslender structure and may result in undesired low-frequency resonantmodes. In the case of electrostatic coupling spring 532, a smallstiffness can be achieved by reducing the electrode area for thein-plane electrostatic coupling spring 532 and shortening the comboverlap length for the out-of-plane electrostatic coupling spring 532.

Because of its stable gain and low noise floor, the circuitryrequirement for the dual mode open-loop system is not complex and simplesignal processing circuitry can be employed. Shown in FIG. 7, a signalprocessing unit 651 in accordance with an embodiment comprising acharge-to-voltage converter 652, a de-modulator 653, and a low-passfilter 654 is coupled to the dual mode system 602. The charge-to-voltageconverter 652 converts the output of the position transducer 640 into avoltage format. The de-modulator 653 down-converts the AC output signalsof the charge-to-voltage converter 652 into a low frequency. Thelow-pass filter 654 reduces the high frequency noise and might serve asan anti-aliasing filter prior digitization. In addition, theelectrostatic coupling spring 632 is deployed with a high voltage bias681 to the sense subsystem 670 resulting in high transduction gain ofthe position transducer 640 mentioned before. The examples noted hereshould not be interpreted to restrict the range of possible signalprocessing circuitry of the dual mode open-loop system. One of ordinaryskill in the art readily recognizes a variety of types of signalprocessing circuitry can be utilized and that use would be within thespirit and scope of the present invention.

Embodiments for in-plane and out-of-plane dual mode open-loop systemsare shown in FIGS. 8 and 9.

For an in-plane dual mode open-loop system 702, a drive point 710 and asense point 720 are mechanically anchored to a substrate 701 through aplurality of beams 711, 712 and 721, 722, respectively.

In the drive subsystem 760, an AC current generated by the driveelectronics 80 of the FIG. 1 passes through a plurality of beams 711, adrive point 710, and a plurality of beams 712. In the presence of amagnetic field vector in X-axis, the Lorentz force generates adistributed actuation force in Z-axis along a drive beam 716 and thetotal force can be modeled asF _(Lorentz,Z)=∫(I _(Y) ×B _(X))dl  (9)where the I_(Y) is the current vector in Y-axis and B_(X) is themagnetic field vector in the X-axis.

The Lorentz force actuates the drive subsystem 760 and the drive point710 and generates an out-of-plane displacement in Z-axis which isproportional to the magnetic field density. An out-of-planeelectrostatic coupling spring 732 transfers the displacement of thedrive point 710 to the sense point 720 via its mechanical stiffness andactuates the sense subsystem 770 into a rotational motion in Y-axis. Therotational motion of the sense subsystem 770 is detected by parallelplate capacitive transducers 740 located beneath the sense subsystem 770as the position transducer 740 and the electrical signals from theposition transducer 740 is further processed by the signal processingunit 50 shown in FIG. 1.

For an out-of-plane dual mode open-loop system 802, a drive subsystem860 and a sense subsystem 870 are mechanically anchored to a substrate801 through a plurality of beams 811, 812, 821, and 822, respectively.

In the drive subsystem 860, an AC current generated by the driveelectronics 80 of the FIG. 1 passes through a plurality of beams 811, adrive point 810, and a plurality of beams 812. In the presence of amagnetic field vector in Z-axis, the Lorentz force generates adistributed actuation force in X-axis along a plurality of beams 811 and812. The total force may be modeled as (10)F _(Lorentz,X)=∫(I _(Y) ×B _(Z))dl  (10)where the I_(Y) is the current vector in Y-axis and B_(Z) is themagnetic field vector in the Z-axis. The Lorentz force actuates thedrive point 810 and generates an in-plane displacement in X-axis and themagnitude of the displacement is proportional to the magnetic fielddensity. The motions of the drive subsystem 860 and the sense subsystem870 are coupled through an in-plane electrostatic coupling spring 832.

The in-plane electrostatic coupling spring 832 transfers thedisplacement of the drive point 810 to a sense point 820 via itsmechanical stiffness and actuates the sense subsystem 870 and the sensepoint 820. The motion of the sense subsystem 870 is detected byinterdigitated comb capacitive transducers 840 as a position transducerand the electrical signals from the position transducer 840 is furtherprocessed by the signal processing unit 50 shown in FIG. 1.

A Multi-axis Magnetic Sensing System

A multi-axis magnetic sensing system 902 can be constructed from thedisclosed open-loop dual mode Lorentz-force magnetic sensing devices andis shown in FIG. 10. Magnetic field sensors 902 a, 902 b, and 902 c aredeployed for sensing x-axis, y-axis, and z-axis portion of the magneticfield vector, respectively.

The Z-axis micromachined magnetic field sensor 902 c, wherein the Z-axismicromachined magnetic field sensor comprises a drive subsystem, thedrive subsystem comprises a plurality of beams, and at least one anchorconnected to the substrate; a mechanism for providing an electricalcurrent through the drive subsystem in a X-Y plane; and Lorentz forceacting on the drive subsystem in the X-Y plane in response to a magneticfield along a Z axis; a sense subsystem, the sense subsystem comprises aplurality of beams, and at least one anchor connected to the substrate;wherein the sense subsystem moves in the X-Y plane; a coupling springbetween the drive subsystem and the sense subsystem which causes motionof the sense subsystem in response to the magnetic field; and a positiontransducer to detect the motion of the sense subsystem.

The X-axis micromachined magnetic field sensor 902 a, wherein the X-axismicromachined magnetic field sensor comprises a drive subsystem, thedrive subsystem comprises a plurality of beams, and at least one anchorconnected to the substrate; a mechanism for providing an electricalcurrent through the drive subsystem in the X-Y plane; and Lorentz forceacting on the drive subsystem along the Z-axis in response to a magneticfield along the X-axis; a sense subsystem, the sense subsystem comprisesa plurality of beams, and at least one anchor connected to thesubstrate; wherein the sense subsystem moves partially along the Z-axis;a coupling spring between the drive subsystem and the sense subsystemwhich causes motion of the sense subsystem in response to the magneticfield; and a position transducer to detect the motion of the sensesubsystem.

The Y-axis micromachined magnetic field sensor 902 b, wherein the Y-axismicromachined magnetic field sensor comprises a drive subsystem, thedrive subsystem comprises a plurality of beams, and at least one anchorconnected to the substrate; a mechanism for providing an electricalcurrent through the drive subsystem in the X-Y plane; and Lorentz forceacting on the drive subsystem along the Z-axis in response to a magneticfield along the Y axis; a sense subsystem, the sense subsystem comprisesa plurality of beams, and at least one anchor connected to thesubstrate; wherein the sense subsystem moves partially along the Z-axis;a coupling spring between the drive subsystem and the sense subsystemwhich causes motion of the sense subsystem in response to the magneticfield; and a position transducer to detect the motion of the sensesubsystem. The method of establishing an open-loop dual modeLorentz-force magnetic sensing device noted here should not beinterpreted to restrict the range of possible multi-axis magneticsensing systems. To form a multi-axis magnetic sensing system, theproposed open-loop dual mode Lorentz-force magnetic sensing device canbe integrated with magnetic sensors consisting of Hall sensors,magnetoresistive sensors, magneto-diode sensors, magneto-transistors,fluxgates, magneto-impedance sensors, magneto-optical sensors, andMAGFETs.

Self-Test Function

A self-test function is important and is commonly implemented as acurrent-carrying coil to generate magnetic fields through the AmpereLaw. However, it consumes significant power as high as severalmilli-watt. For example, 3-axis digital compass HMC5843 manufactured byHoneywell Corporation consumes 3 mA current to generate a 30 uT magneticfield (comparable to the earth magnetic field).

FIG. 11A is an embodiment of the self-test function which includes aself-test actuator 1014 which connects to a drive subsystem 1060. In theself-test mode, the self-test actuator 1014 excites the drive subsystem1060 and generates output via the first transfer function 210 of FIG. 3.The self-test actuator 1014 can be designed with mechanisms comprisingparallel plate capacitive transducers, interdigitated comb capacitivetransducers, electro-thermal actuators, and piezoelectric actuators.

An embodiment of parallel plate capacitive transducers 1014 a and 1014 bas the self-test actuator 1014 is shown in FIG. 11B. In the self-testmode, the voltage of either capacitive transducers 1014 a or 1014 bbiased at a self-test voltage which is different from the voltage of thedrive subsystem 1060. The voltage difference results in an electrostaticforce which actuates the dual mode open-loop system 1002. The method ofestablishing a self-test actuator 1014 for a magnetic sensing devicenoted here should not be interpreted to restrict the range of possibleapplication. An embodiment of the self-test function with the self-testactuator 1014 for a single mode magnetic sensing device 1002 is shown inFIG. 11C.

In the case of the implementation of a parallel plate capacitiveactuator as the the self-test actuator 1014, the equivalent magneticfield density is

$\begin{matrix}{B_{Equivalent} = {\frac{\frac{ɛ\; A_{ST}}{g_{ST}^{2}}V_{D}}{\int_{L_{D}}{I \cdot \ {\mathbb{d}l}}}V_{ST}}} & (11)\end{matrix}$where A_(ST), g_(ST), V_(D), V_(ST), I, and L_(D) are the parallel platearea, parallel plate gap, the DC voltage of the drive subsystem 1060, aself-test voltage, current flow across the drive subsystem 1060 and theeffective beam length of the drive subsystem 1060. With 30 um×2.8 um ofthe parallel plate area, 5 um of the capacitive gap, biased at 1 voltageof the drive subsystem 1060, supplying 0.5 voltage of self-test voltageacross the parallel plate creates a equivalent force of 30 uT magneticdensity when the drive subsystem 1060 is within 500 um of the effectivebeam length of the drive subsystem 1060 and 1 mA of the driving ACcurrent.

The advantage of the proposed self-test function in terms of loweringpower consumption is dramatic. Compared with several milliwatts of thepower consumption for the prior art, the power consumption for voltageacross the capacitor of the self-test actuator 1014 is negligible.

The examples noted here should not be interpreted to restrict the rangeof possible integration of the dual mode or single mode open-loop systemwith current-carrying coils to generate a magnetic field via the AmpereLaw. FIGS. 11D and 11E are embodiments of a dual mode open loop systemand a single mode open loop system respectively that includes currentcarrying coils 1014 c. The current-carrying coils 1014 c each generate amagnetic field along the third axis which interacts with the drivesubsystem resulting in Lorentz force acting on the drive subsystem alongthe second axis.

To overcome process variation, frequency tuning is critical to ensurehigh signal to noise ratio. As before mentioned with reference to FIG.4, sensitivity is proportional to the first transfer function 210 andsmall frequency separation between two resonant peaks labeled as 220 and230 leads to high sensitivity. In addition, the second transfer function310 is related to the total Brownian noise. Ideally, by operating thedual mode open-loop system 2 (FIG. 1) at the anti-resonance ω_(N) 350 ofthe transfer function 310, the Brownian noise output is minimized.

However, because of manufacturing variation resulting in changes instructure stiffness, the frequency separation two resonant peaks 220 and230 could be widened leading to low sensitivity and the anti-resonanceω_(N) 350 of the transfer function 310 is pulled close to the one of theresonant peaks leading to high sensitivity variation. These outcomesdegrade signal to noise ratio.

Stiffness Tuning Block

A stiffness tuning block 1118 for a dual mode open-loop system 1102 isproposed to address the effect of stiffness variation, shown in FIG.12A. Shown in Equation (6), the frequency separation between 220 and 230is a function of the stiffness of a drive subsystem 1160, a sensesubsystem 1170, and a coupling spring 1130. Thus, the transfer function210 and 310 and the resonant peaks 220 and 230 can be tuned by changingeither the stiffness of the drive subsystem 1160, or the sense subsystem1170, or the coupling spring 1130. An embodiment of a stiffness tuningblock 1118 to tune the stiffness and frequency of the sense subsystem1170 for a dual mode open-loop system 1102 is shown in FIG. 12A. Anexample of electrostatic parallel plates 1118 a and 1118 b as thestiffness tuning block is shown in FIG. 12B. The tuning stiffness Δk_(T)of the electrostatic parallel plates 1118 a and 1118 b can be modeled asEquation (7) where A, g, and V are the area of parallel plates 1118 aand 1118 b, electrode gap, and bias voltage across the parallel plates1118 a and 1118 b. Thus, during calibration, the bias voltage across theparallel plates can be tuned and the tuning stiffness Δk_(T) from thestiffness tuning block changes the stiffness of the sense subsystem 1170and ensure the frequency separation and the anti-resonance ω_(N) 350 ofthe transfer function 310 matches the design values.

The application of the stiffness tuning block 1118 is not limited to theembodiment shown in FIGS. 12A and 12B. Other tuning mechanism such asthermal actuators or piezoelectric actuators can be applied as animplementation of the stiffness tuning block 1118. In addition, asimilar frequency tuning block can be applied to change the stiffness ofthe drive subsystem 1160 or the coupling spring 1130. An example of thestiffness tuning block 1118 for single mode magnetic field sensingsystem 1102′ is shown in FIG. 12C.

Offset Cancellation

Resonant sensing systems are prevalent for their high sensitivity andthe application ranges from magnetic field sensors, gyroscopes,accelerometers, chemical sensors, biosensors, pressure sensors, to forcesensors. However, for resonant sensing systems with AC drive signals,the interactions between drive signals and the surrounding parasiticcapacitors generates electrostatic offset forces and thus createsunwanted feedthrough outputs, or called offset. In general, the offsetis larger than the signal generated by the field of interest. Thus, theoffset variation may limit the minimum detectable signal level and thedesign to mitigate offset is critical.

For resonant magnetic field sensors, the first offset source is theelectrostatic force from the shield structure. Shown in FIG. 13A, a dualmode open-loop system 1202 with shield structure comprising anelectrical source 1282, and at least one electrode, as 1213R and 1213L,to reduce the electrostatic offset force along the second axis generatedby parasitic capacitors around a drive subsystem 1260. The equivalentoffset from the electrostatic force generated by the shield electrodesis expressed

$\begin{matrix}{B_{{offset},{SH}} \propto {( {\frac{H_{{SH},L}}{g_{{SH},L}^{2}} - \frac{H_{{SH},R}}{g_{{SH},R}^{2}}} )\Delta\; V_{D,{SH}}{\int_{L_{D}}{\frac{\Delta\; V_{A\; C}}{I} \cdot \ {\mathbb{d}l}}}}} & (12)\end{matrix}$where H_(SH), g_(SH), ΔV_(D,SH), and ΔV_(AC) are the height of theshield electrode, the gap of the shield electrode, and the DC voltagedifference between the drive subsystem 1260 and the electrical source1282 of the shield electrode, the AC voltage difference between thedrive subsystem 1260 and the electrical source 1282 of the shieldelectrode. The label L or R of the height and the gap of the shieldelectrode indicates the left-side 1213L or right-side of the shield1213R, respectively.Offset Mitigation with Shield Structure with Force-BalancedCharacteristic

The force-balanced shield structure is implemented with two equal shieldheights H_(SH) and gaps g_(SH) of the left-side shield 1213L andright-side shield 1213R. Thus, the electrostatic forces generated by theleft shield 1213L is balanced by the force generated by the right-sideshield 1213R and the first term of Equation (11) is cancelled in thefirst order resulting in small offset.

Offset Mitigation with Voltage Tuning of Shield Structure

However, due to manufacturing imperfection of mismatched shield heightor shield gap, there is residue offset even with the design of theforce-balanced shield structure. Additional cancellation can be achievedby tuning the voltage of the shield structure to null the DC voltagedifference ΔV_(D,SH) between the drive subsystem 1260 and the shieldelectrodes 1213R and 1213L. Thus, the second term of Equation (12) iscancelled resulting in small offset.

Offset Mitigation with Balanced AC Voltage Distribution

Additional cancellation can be achieved by designing the distribution ofthe AC voltage of the drive subsystem 1260. The effective beam lengthL_(D) of the drive subsystem 1260 is the length of a plurality of beams1211 and 1212. By having the same length of a plurality of beams 1211and 1212 and applied same magnitude but opposite polarity of AC voltagedistribution across the drive subsystem 1260, the third term of equation(12) is cancelled resulting in small offset.

The examples noted here should not be interpreted to restrict the rangeof possible application of the offset mitigation technique with shieldstructure. An embodiment of the offset mitigation with shield structuresfor a single mode system 1202 with shield structures 1213R and 1213Lacross a drive subsystem 1260, as shown in FIG. 13B.

A second offset source is the electrostatic force through theelectrostatic coupling spring. The second offset source results from anAC electrostatic force generated by AC voltage across the electrostaticcoupling spring element. The offset can be modeled as

$\begin{matrix}{B_{{offset},C} \propto {( {\frac{A_{C,L}}{g_{C,L}^{2}} - \frac{A_{C,R}}{g_{C,R}^{2}}} )\Delta\; V_{DS}\Delta\; V_{{A\; C},C}}} & (13)\end{matrix}$where A_(C), g_(C), ΔV_(DS), and ΔV_(AC,C) are the area of the couplingspring, the gap of the coupling spring, the DC voltage differencebetween the drive subsystem 1260 and the sense subsystem 1270, the ACvoltage difference at the coupling point between the drive subsystem1260 and the sense subsystem 1270, and the label L or R of the area andthe gap of the coupling spring 1230 indicates the two side of thecoupling spring.Offset Mitigation with Force-balanced Coupling Spring

Shown in FIG. 6, a force-balanced coupling spring 532 for in-planecoupling is implemented with two equal coupling area and gaps of 532 aand 532 b. Thus, the electrostatic force generated by the left electrode532 a is balanced by the force generated by the right-side electrode 532b and the first term of Equation (13) is cancelled in the first orderresulting in small offset. The same force-balance technique can beapplied to the out-of-plane coupling spring 532.

Offset Cancellation with Mismatch-trimming Actuator

However, due to manufacturing imperfection of mismatched gap, there isresidue offset even with force-balanced coupling spring where theresidue offset is proportional to the mismatch. Shown in FIG. 14A, amismatch-trimming actuator (MTA) 1315 is implemented to move a sensesubsystem 1370 in order to reduce the gap mismatch. The MTA 1315 can bedesigned with mechanisms comprising parallel plate capacitivetransducers, interdigitated comb capacitive transducers, electro-thermalactuators, and piezoelectric actuators.

An embodiment of parallel plate capacitive transducers as MTA 1315 isshown in the FIG. 14B. The displacement of the electrostatic actuationcan be expressed as

$\begin{matrix}{{\Delta\; x_{MTA}} \propto {\frac{A_{MTA}}{g_{MTA}^{2}K_{S}}\lbrack {( {V_{s} - V_{{MTA},{15a}}} )^{2} - ( {V_{s} - V_{{MTA},{15b}}} )^{2}} \rbrack}} & (14)\end{matrix}$where A_(MTA), g_(MAT), V_(MTA), V_(S) and K_(S), are the area of theMTA, the gap of the MTA, the applied voltage at MTA, the voltage of asense point 1320, and the stiffness of the sense subsystem 1370. In amismatch case that the gap 1332 b of the in-plane coupling spring 1332is larger than gap 1332 a, by applying the voltages of MTA 1315 a higherthan MTA 1315 b and assuming Vs is larger than both voltages, theelectrostatic actuation force of MTA 1315 b is larger than that of theMTA1315 a and moves the sense node 1320 and the sense subsystem 1370toward the MTA 1315 b. Thus, the mismatch can be null. With the sameconcept, the MTA technique can be applied to out-of-plane couplingspring 1332 with the implementation of such as out-of-plane parallelplate capacitive transducers.

Furthermore, a mismatch-trimming actuator (MTA) 1315 can be applied toreduce the shield gap mismatch. Shown in FIG. 14C, a mismatch-trimmingactuator (MTA) 1315 is implemented to move the drive subsystem 1360 inorder to reduce the shield gap mismatch. Thus, the first term ofEquation 12 for offset caused by shield structure can be furtherreduced.

Offset Cancellation with AC Ripple Detection

The ideal coupling location for the electrostatic coupling spring is atAC ground because the AC voltage difference ΔV_(AC,C) is zero at thelocation. Therefore, the offset generated by electrostatic force acrossthe electrostatic coupling spring is zero shown in Equation (13).However, due to manufacturing imperfection of resistance variation,there is ripple at the coupling point resulting in residue offsetproportional to the magnitude of the ripple. Therefore, a structure todetect the ripple is proposed which can be connected to electroniccircuits to null the ripple or to compensate for the ripple.

Shown in FIGS. 15A and 15B, the ripple detection at a drive point 1410can be realized as the resistive type or capacitive type, respectively.Shown in FIG. 15A, a resistive type detection 1417 connects the drivepoint 1410 to an anchor 1419 wherein the ripple of the drive point 1410causes a current flow through the plurality of beams resulting in thevoltage variation at an anchor 1419. Thus, the ripple of the drive point1410 can be measured at the anchor 1419 by the ripple detectionmechanism. Shown in FIG. 15B, a capacitive type detection 1418 connectsa drive point 1410 to an anchor 1419 wherein the ripple of the drivepoint 1410 causes a charge variation across the capacitor resulting inthe current variation at the anchor 1419. Thus, the ripple of the drivepoint 1410 can be measured at the anchor 1419 by the ripple detectionmechanism.

Examples of micromachined magnetic field sensors with the self testactuator, MTA, balanced shield, and resistive type ripple detectionmechanism are shown in FIGS. 16A and 16B. FIG. 16A shows an x-axismicromachined magnetic field sensor 1502. An AC current generated by adrive electronic passes through a plurality of beams 1511 and 1512 andestablishes a current vector in a Y-axis at a drive point 1510. Adistributed Lorentz force is generated in a z-axis in the presence of amagnetic field in an x-axis. The sensor comprises an electrostaticactuator 1514 as a self test actuator to generate forces in the z-axisthe same direction as the distributed Lorentz force, an electrostaticactuator 1515 as a MTA moving the sense subsystem along the z-axis, abalanced shield electrode 1513 located beneath a drive subsystem 1560 tonull electrostatic offset torque about the y-axis, and a plurality ofbeams 1517 as a resistive type ripple detection mechanism and an anchor1519 to sense the ripple voltage of the drive point 1510.

A Z-axis micromachined magnetic field sensor 1602 is shown in FIG. 16B.An AC current generated by a drive electronic passes through a pluralityof beams 1611 and 1612 and establishes a current vector in a Y-axis at adrive point 1610. A distributed Lorentz force is generated in an X-axisof a drive subsystem 1660 in the presence of a magnetic field in aZ-axis. The sensor comprises an electrostatic actuator 1614 as a selftest actuator to generate forces in the x-axis the same direction as thedistributed Lorentz force, an electrostatic actuator 1615 as a MTAmoving the sense subsystem along the x-axis, balanced shield electrodes1613 close to the drive subsystem 1660 to null electrostatic offsetforces in the x-axis, and a plurality of beams 1617 as a resistive typeripple detection mechanism and an anchor 1619 to detect the ripplevoltage of the drive point 1610.

The examples of the offset cancellation techniques in a magnetic fieldsensing system noted here should not be interpreted to restrict therange of possible application of the offset cancellation techniques. Itis advantageous to implement the offset cancellation techniques to otherresonant applications comprising gyroscopes, accelerometers, chemicalsensors, biosensors, pressure sensors, to force sensors.

Advantages

A magnetic field sensor is utilized for MEMS gyroscopes andaccelerometers without major process change. In addition, the sensoraddresses the need of low-power, high-manufacturability, andhigh-sensitivity. The sensor includes the following features.

1. A low cost and open-loop magnetic field resonant magnetic fieldsensor to achieve low power consumption in a system level circuitcompared to a close loop frequency control resonant system.

2. Allows for use of a dual-mode resonating device to accommodate theprocess variation to achieve high manufacturability.

3. Incorporates an electrostatic coupling spring to deploy high biasvoltage at the sensing node. The high bias voltage leads tohigh-sensitivity in the system.

4. Provides a low offset with several offset mitigation techniques toremove offset shift issues.

5. Includes a self-test function with electrostatic actuators to achievelow power consumption comparing to generating magnetic field directlywith 3 mA supply.

-   -   Integration with other motion sensors. By harnessing these        design strategies, a Lorentz force-effect resonant sensor can be        implemented in a way to allow for on-chip integration of        magnetic field sensors with other motion sensors.    -   Although the present invention has been described in accordance        with the embodiments shown, one of ordinary skill in the art        will readily recognize that there could be variations to the        embodiments and those variations would be within the spirit and        scope of the present invention. Accordingly, many modifications        may be made by one of ordinary skill in the art without        departing from the spirit and scope of the appended claims.

What is claimed is:
 1. A micromachined magnetic field sensor comprising:a substrate; a drive subsystem with a plurality of beams flexiblycoupled to the substrate; a mechanism for providing an electricalcurrent through the drive subsystem along a first axis; a magnetic fieldvector along a second axis generates a Lorentz force on the drivesubsystem along a third axis; a position transducer responds to theLorentz force on the drive subsystem; and an offset cancellationmechanism electrostatically coupled to the drive subsystem; wherein theoffset cancellation mechanism comprises at least one shield structurecoupled to the substrate by at least one anchor, wherein the at leastone shield structure comprises a plurality of beams spaced equally onopposite sides of the drive subsystem, wherein the drive subsystem movesrelative to the at least one shield structure.
 2. The micromachinedmagnetic sensor of claim 1, wherein the offset cancellation mechanismcomprises a mismatch-trimming actuator moving the sense subsystem alongthe second axis in order to reduce the gap mismatch of theforce-balanced electrostatic coupling springs.
 3. The micromachinedmagnetic sensor of claim 2, wherein the mismatch-trimming actuatorcomprises any of parallel plate capacitive transducers, Interdigitatedcomb capacitive transducers, electro-thermal actuators, andpiezoelectric actuators.
 4. The micromachined magnetic sensor of claim1, wherein the offset cancellation mechanism comprises a resistive typeripple detection comprising a plurality of beams and at least one anchorwhere the resistive type detection connects the drive point to an anchorfor detecting the ripple of the drive point.
 5. The micromachinedmagnetic sensor of claim 1, wherein the offset cancellation mechanismcomprises a capacitive type ripple detection comprising a capacitorsupported by a plurality of beams and at least one anchor where thecapacitor connects the drive point to an anchor for detecting the rippleof the drive point.
 6. The micromachined magnetic field sensor of claim1, wherein the at least one shield structure is biased with a DCvoltage.
 7. The micromachined magnetic field sensor of claim 6, whereinthe DC voltage is adjusted for a minimum offset.
 8. The micromachinedmagnetic field sensor of claim 1, wherein the position transducerresponds to motion of the drive subsystem relative to the substrate. 9.The micromachined magnetic field sensor of claim 1, wherein the positiontransducer comprises a capacitive transducer, wherein the capacitivetransducer is an interdigitated comb capacitive transducer or a parallelplate capacitive transducer.
 10. A micromachined magnetic field sensorcomprising: a substrate; a drive subsystem with a plurality of beamsflexibly coupled to the substrate; a mechanism for providing anelectrical current through the drive subsystem along a first axis; amagnetic field along a second axis generates a Lorentz force on thedrive subsystem along a third axis; a position transducer responds tothe Lorentz force acting on the drive subsystem; and an offsetcancellation mechanism electrostatically coupled to the drive subsystem,wherein the offset cancellation mechanism comprises force-balancedelectrostatic coupling springs comprising two equal coupling capacitorswherein a first electrostatic force generated by a first capacitor isbalanced by a second electrostatic force generated by a secondcapacitor.
 11. The micromachined magnetic field sensor of claim 10,wherein the force-balancing electrostatic coupling springs connect tothe drive subsystem at a point with zero AC voltage.
 12. A micromachinedmagnetic field sensor comprising: a substrate; a drive subsystem with aplurality of beams flexibly coupled to the substrate; a mechanism forproviding an electrical current through at least one of the plurality ofbeams of the drive subsystem along a first axis; a magnetic field alonga second axis generates a Lorentz force on the drive subsystem along athird axis; a position transducer responds to the Lorentz force actingon the drive subsystem; and an offset cancellation mechanismelectrostatically coupled to the at least one of the plurality of beamsof the drive subsystem, wherein the offset cancellation mechanismcomprises at least one shield structure coupled to the substrate by atleast one anchor, wherein the at least one shield structure is biasedwith a DC voltage to reduce electrostatic offset forces acting on the atleast one beam.
 13. The micromachined magnetic field sensor of claim 12,wherein the position transducer comprises a capacitive transducer,wherein the capacitive transducer is an interdigitated comb capacitivetransducer or a parallel plate capacitive transducer.